U.S. Secretary of State Hillary Clinton said Thursday that Iran's disappointing approach to big-power offers on its nuclear program leaves little choice but to put more pressure on Tehran. Clinton held meetings on Iran with several foreign ministers on the sidelines of the London conference on Afghanistan.

Despite the day's focus on Afghanistan, the Iran nuclear issue figured prominently in Clinton's series of bilateral meetings, including what she said was a very constructive conversation with Chinese Foreign Minister Yang Jiechi.

A senior U.S. official said Iran was the main focus of the meeting with Yang. He said that while Beijing has concerns that new U.N. sanctions against Tehran could endanger prospects for a negotiated solution to the nuclear issue, Yang did not say no to the idea.

Clinton told reporters that China is an engaged and very active member of the informal grouping of the five permanent U.N. Security Council member countries and Germany - the P5+1 - that has conducted nuclear diplomacy with Iran.

Clinton said she and Yang agreed to step up bilateral dialogue on possible measures against Iran.

"We shared some of our thoughts with our Chinese counterparts. We also set up some additional opportunities for expert consultations," she declared. "We made it clear to everyone with whom I spoke today and yesterday that our efforts to apply pressure on Iran are not meant to punish the Iranian people. They are meant to change the approach that the Iranian government has taken toward its nuclear program," she said.

Clinton said Iran's refusal to accept a U.N. backed compromise proposal on uranium enrichment and disclosure that it was building a secret enrichment plant near Qom have only added to doubts about Tehran's assertions that its nuclear program is peaceful.

"In response to these questions, the Iran government has provided a continuous stream of threats to intensify its violations of international nuclear norms," said Clinton. "Iran's approach leaves us with little choice but to work with our partners to apply greater pressure in the hopes that it will cause Iran to reconsider its rejection of diplomatic efforts with respect to its nuclear ambitions," she added.

Iran has been a participant in past international conferences on Afghan security, but it refused to send a representative to the London conference.

British Foreign Secretary David Miliband said the decision was puzzling, given Iranian Foreign Minister Manouchehr Mottaki's expressed concerns about instability in Afghanistan.

"He's talked to me about the damage and the danger that instability in Afghanistan poses in Iran," he said. "He's talked to me about the damage and danger of drugs coming from Afghanistan to Iran. That's one reason among a number why we thought it was right to invite Iran to participate in this conference, just like any other country. We think it's deeply regrettable as well as inexplicable that they failed to attend," said the British foreign secretary.

Miliband says Iran's failure to attend the London conference calls into question its statements that it wants to play a positive role on Afghan security, and that other countries "will draw their own conclusions" about the seeming contradiction.